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<h3><a name="SECTION001614119000000000000000"></a><a name="ctypes-surprises"></a>
<br>
14.14.1.19 Surprises
</h3>

<p>
There are some edges in <code>ctypes</code> where you may be expect something
else than what actually happens.

<p>
Consider the following example:
<div class="verbatim"><pre>
&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class POINT(Structure):
...     _fields_ = ("x", c_int), ("y", c_int)
...
&gt;&gt;&gt; class RECT(Structure):
...     _fields_ = ("a", POINT), ("b", POINT)
...
&gt;&gt;&gt; p1 = POINT(1, 2)
&gt;&gt;&gt; p2 = POINT(3, 4)
&gt;&gt;&gt; rc = RECT(p1, p2)
&gt;&gt;&gt; print rc.a.x, rc.a.y, rc.b.x, rc.b.y
1 2 3 4
&gt;&gt;&gt; # now swap the two points
&gt;&gt;&gt; rc.a, rc.b = rc.b, rc.a
&gt;&gt;&gt; print rc.a.x, rc.a.y, rc.b.x, rc.b.y
3 4 3 4
&gt;&gt;&gt;
</pre></div>

<p>
Hm. We certainly expected the last statement to print <code>3 4 1 2</code>.
What happended? Here are the steps of the <code>rc.a, rc.b = rc.b, rc.a</code>
line above:
<div class="verbatim"><pre>
&gt;&gt;&gt; temp0, temp1 = rc.b, rc.a
&gt;&gt;&gt; rc.a = temp0
&gt;&gt;&gt; rc.b = temp1
&gt;&gt;&gt;
</pre></div>

<p>
Note that <code>temp0</code> and <code>temp1</code> are objects still using the internal
buffer of the <code>rc</code> object above. So executing <code>rc.a = temp0</code>
copies the buffer contents of <code>temp0</code> into <code>rc</code> 's buffer.  This,
in turn, changes the contents of <code>temp1</code>. So, the last assignment
<code>rc.b = temp1</code>, doesn't have the expected effect.

<p>
Keep in mind that retrieving subobjects from Structure, Unions, and
Arrays doesn't <em>copy</em> the subobject, instead it retrieves a wrapper
object accessing the root-object's underlying buffer.

<p>
Another example that may behave different from what one would expect is this:
<div class="verbatim"><pre>
&gt;&gt;&gt; s = c_char_p()
&gt;&gt;&gt; s.value = "abc def ghi"
&gt;&gt;&gt; s.value
'abc def ghi'
&gt;&gt;&gt; s.value is s.value
False
&gt;&gt;&gt;
</pre></div>

<p>
Why is it printing <code>False</code>?  ctypes instances are objects containing
a memory block plus some descriptors accessing the contents of the
memory.  Storing a Python object in the memory block does not store
the object itself, instead the <code>contents</code> of the object is stored.
Accessing the contents again constructs a new Python each time!

<p>

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